Bacterial pathogens must avoid clearance by the immune system to establish infection, yet many mechanisms of bacterial immune subversion remain undefined. A number of bacterial pathogens subvert pathways of the innate immune system, but how bacterial pathogens overcome pathways of the adaptive immune system is not well understood. T cells are a key component of the adaptive immune system and are required for protective immunity against many bacterial pathogens. Salmonella enterica serovar Typhimurium (S. typhimurium) are pathogenic bacteria that inhibit the response of T cells directly, but the factor responsible for this inhibition has not been identified. We recently showe that S. typhimurium inhibit T cell responses by producing L- asparaginase II, which hydrolyzes L-asparagine. L-asparaginase II is necessary and sufficient to suppress T cell blastogenesis, cytokine production and proliferation, and to down-modulate expression of the T cell receptor. Furthermore, S. typhimurium-induced inhibition of T cells in vitro is prevented upon addition of exogenous L-asparagine. S. typhimurium lacking the L-asparaginase II gene are unable to inhibit T cell responses and exhibit attenuated virulence in vivo. L-asparaginases are used clinically to treat acute lymphoblastic leukemia, yet production of L-asparaginase II by pathogenic bacteria has been unrecognized as a mechanism of microbial immune subversion. The research proposed in this application will 1) determine the mechanism by which L-asparaginase II produced by S. typhimurium inhibits T cell responses and mediates virulence, and 2) determine the role of L-asparaginase II in the pathogenesis of, and host response to, infection with S. typhimurium. Completion of the proposed research will provide answers to important mechanistic questions and will provide a vastly enhanced perspective on the role of T cells in protective immunity against S. typhimurium. Given that the L-asparaginase II gene is highly conserved in Gram-negative bacteria and has been shown to contribute to virulence of several important human pathogens, our findings may extend well beyond S. typhimurium. Insights from the proposed research will have fundamental implications for understanding host interactions with bacterial pathogens and could lead to the development of new broad- spectrum therapeutic approaches and preventive measures to overcome bacterial infectious diseases.